The present invention relates to a method for preparing metal powder, and in detail to a method for preparing fine and spherical metal powder having a narrow particle size distribution.
There have been several methods for preparing metal powder, one of which is known as an atomizing method that is a way accompanied with a cooling medium (or an atomizing medium) blowing to a melt metal flow to efficiently prepare metal powder. The atomizing method is generally classified into a gas atomizing method using a gas cooling medium and a liquid atomizing method using a liquid cooling medium.
As an example of gas atomizing method, a method has been known to utilize a nozzle disclosed in U.S. Pat. No. 1,659,291 and U.S. Pat. No. 3,235,783. While gas jet discharged from the nozzle according to the gas atomizing method can not be watched, observation by Schlieren photography can support that it flows to expand monotonously. It is considered that gas jet is a compressible fluid to adiabatically expand just after discharged from the nozzle. Since an adiabatic expansion makes the energy density of the gas jet decrease suddenly, it is difficult to efficiently obtain fine metal powder by means of the gas atomizing method. Thusly prepared metal powder has a broad particle size distribution. Also, the gas atomizing method is accompanied with another problem that the atmosphere may engulf the gas jet to blow up the melt metal.
The gas used as a cooling medium however has a relatively small cooling ability so that the melt metal drop dispersed by the gas jet may make solidification after changing itself into a spherical shape. Therefore the metal powder prepared according to the gas atomizing method has a generally spherical shape.
The nozzle disclosed in above-mentioned U.S. Pat. No. 1,659,291 and U.S. Pat. No. 3,235,783 is provided with gas inlets in the tangential direction of the nozzle and blades inside the nozzle to direct the discharged gas jet into the direction similarly inclined with respect to the center of the nozzle. It is considered that this inclined direction prevents the atmosphere from engulfing the gas jet so that the melt metal may not be blown up.
On the other hand, There have been known such liquid atomizing methods as V-jet type liquid atomizing method (shown in FIG. 11(a) or FIG. 11(b)) characterized in that the liquid jet converges in a line, conical jet type liquid atomizing method (shown in FIG. 11(c)) characterized in that liquid jet converges in one point, or pencil jet type liquid atomizing method (shown in FIG. 11(d)) characterized in that the liquid jet discharged from pencil jet type nozzle parts 14 converges in one point.
Since the cooling medium used in a liquid atomizing method is an incompressible fluid, the energy density of the liquid jet for dispersing the melt metal flow 6 is much larger than that of a gas jet. Therefore, the liquid atomized metal powder is finer than the gas atomized metal powder.
However, prior art liquid atomizing methods are accompanied with a problem that the liquid jet converges or collides in a line or in one point. Thus, the dispersed melt metal drops before solidification have to concentrate to the vicinity of the focus and cross the liquid jet violently to thereby be cooled suddenly. Therefore, the dispersed melt metal drops contact and adhere to each other in the form of cluster so that the obtained metal powder has an irregular shape and a broad particle size distribution including coarse metal powder.
Thus, if demanding metal powder having a spherical shape and a narrow particle size distribution, another separation or mechanical treatment must be added which thereby raises its preparation cost.
There have been several improvements to solve the above-mentioned problems in the liquid atomizing method.
One of the improvements is that V-jet or conical jet converges while having a focus of a smaller vertical angle to thereby decrease the collision energy of the liquid jet so as to decrease the deformation of the dispersed metal drops. However actually obtained metal powder does not have a spherical shape. And since this improvement makes the distance between the nozzle and the focus longer, larger energy loss occurs so that the obtained metal powder may include coarse metal powder having a broader particle size distribution.
Several improvements for conical jet type liquid atomizing method are disclosed in Japanese patent No. 552,253 (Japanese Patent Publication No. 43-6,389), Japanese Patent Publication No. 3-55,522 and Japanese Patent Publication No. 2-56,403. According to the invention disclosed in Japanese Patent Publication No. 2-56,403, a cooling medium is introduced in the tangential and the normal direction of the nozzle for discharging the liquid jets. If the liquid jet discharged has a condition of making a hole, only a coarse metal powder is prepared.
Another improvement is disclosed in Japanese Patent Publication No. 53-16,390, which is provided with an exhaust pipe in the under surface for making the liquid jet turbulent to promote the efficiency for dispersing the melt metal flow. According to the improvement, the melt metal flow contacts violently with the turbulent liquid jet to prepare fine metal powder, which is however not spherical shaped.
An annular nozzle of swirling type is disclosed in Japanese laid open patent publication No. 1-123012, which discharges a cooling medium surrounding the melt metal flow in the form of a hyperboloid of one sheet. The liquid jet is discharged from the annular nozzle for dispersing to successively shave off the circumference of the melt metal flow passing through the constricted part of the hyperboloid of one sheet. Thus, this nozzle prevents dispersed melt metal drops from adhering to each other to thereby prepare fine and spherical metal powder. However since the efficiency for dispersing the melt metal flow is very low, a part of the melt metal flow is not dispersed to pass through the constricted part of the hyperboloid of one sheet so as to generate a coarse metal powder. Therefore, metal powder having a narrow particle size distribution can not be actually prepared by the annular nozzle disclosed in Japanese laid open patent publication No. 1-123,012.
This invention provides a technique for efficiently preparing finer and more spherical metal powder having a narrower particle size distribution than that of prior art liquid atomizing method.
Present inventors have considered various alternatives in order to overcome above problems and have accomplished the present inventions. There is provided a method for preparing metal powder by means of blowing a cooling liquid toward a flowing down melt metal flow characterized that the cooling liquid is successively discharged downwardly from an annular nozzle toward the melt metal flow for surrounding it in the form of a hyperboloid of one sheet, wherein the annular nozzle is provided with a hole through which the melt metal flow may pass, and that the hyperboloid of one sheet has a pressure reduced by 50xcx9c750 mmHg at the neighborhood of the constricted part inside the hyperboloid of one sheet.
Thus, above mentioned problems are overcome by discharging liquid jet toward a flowing down melt metal flow in the form of a hyperboloid of one sheet and generating a remarkably large pressure difference inside the hyperboloid of one sheet. There are several ways to reduce the pressure inside the hyperboloid of one sheet. For example, it may be reduced by disposing an exhaust pipe at the lower part of the annular nozzle described hereinafter, using a chamber having a relatively small inner volume, or disposing a preferable exhaust apparatus at a chamber.
Followings are detail descriptions about the present inventions.